May have e Might have

Essentially may is "poder" in the present form, and might is in the past form.


But there are other usages, for example in the sentence "many may disagree with me on that...many might disagree with me on that." wouldn´t make much difference when I mean that after my present statement someone has the possibility of disagreeing or not.

It´s almost a fixed expression, it´s usage.
I hope it helps clear up your doubt.

Ah, it´s worth noticing I am not a teacher or native speaker, so let´s wait more definite and more apt answer, you know,from insighters.
As yet, that´s my collaboration and view on the matter.

Esse é um assunto um pouco complexo.

Uma boa resposta em http://grammar.quickanddirtytips.com/may-might.aspx.

Embora seja em inglês, acho que você consegue acompanhar a explicação.

Há também uma simplificação bem útil em http://melhoreseuingles.wordpress.com/2 ... sdeingles/. que se limita ao ponto principal do site anterior.